Climate and recent occurrence of SBB epidemics in Northern Baltic Sea area
Results showed differences in regional climates within the NBSA, which can explain why there has recently been a large scale SBB epidemic in Sweden, on the western side of the NBSA, but not in Estonia or Finland, on the eastern side of NBSA. Since 2000, the extreme rise of average temperature sums well above 1500 DD have favoured reproduction and growth of SBB populations in East Central Sweden (Annila 1968; Baier et al. 2007; Jönsson et al. 2007), where high temperatures have been combined with co-occurrence of acute drought stress (Netherer et al. 2019). Additionally, the results align with the analyses of Aalto et al. (2021), which indicate significant changes in thermal growing seasons since 1950 in North Europe.
Very low HTI values indicate the combination of high temperature sums and low precipitation in East Central Sweden (see Supplement, Fig S2), which have made local Norway spruce forests susceptible to SBB outbreaks (Netherer et al. 2019; Hlásny et al. 2021a). In fact, in the past five years, the HTI values in East Central Sweden have been very low (Fig. S2), even with the standards of the Southern Baltic Sea area in Germany and Poland (Fig. 1), where spruce forests have suffered more chronically from SBB epidemics than in NBSA (see the Introduction).
Although the temperature sums have been equally high both in Southeastern Estonia and in East Central Sweden, the precipitation has been higher in Southeastern Estonia. As a result, spruce forests in Estonia have not suffered from as severe acute drought stress as those in Sweden, which may have saved Estonia from a large-scale SBB epidemic. On the other hand, in Southern Finland, precipitation has been higher and temperature sums much lower than in East Central Sweden. In these conditions, the population growth of SBB has been much lower than in East Central Sweden or Estonia, and drought stress of spruce trees is less severe than in Sweden.
As mentioned, the average annual temperature sum has achieved the level of 1500 DD, which can be considered a necessary precondition for the occurrence of SBB epidemics (Jönsson et al. 2011) in the early 2000s in East Central Sweden and Southeastern Estonia. A similar substantial increase in thermal sums after 2000 and northward movement of 1500 DD isocline have been reported in European Russia (Romashkin et al. 2020). However, in Southern Finland, where some of the years have been warm (> 1500 DD), years with unfavourable weather for the reproduction of SBB have been more common, limiting the SBB population growth. For example, the heat and storms in 2010 and 2011 initiated an SBB outbreak epidemic in Southeastern Finland from 2012 to 2014, which faded down after cold and wet summers from 2015 to 2017 (J.A. Pulgarin et al., unpublished).
Future climate projections predict the average regional changes in precipitation and temperature. However, in stand level, the realised drought risk is a result of precipitation and is affected by site factors, including topography, soil texture, and stoniness (Kosunen et al. 2019). Variation in groundwater level also needs to be accounted for. According to Netherer et al. (2019), SBB damage risk is higher in sites suffering from acute water stress, whereas spruce trees growing in chronically dry soils may be less favoured as host trees. Furthermore, the physiological interactions among drought, carbon-based defences of a host tree, pathogens, and bark beetle outbreaks are complicated and uncertain – there are both positive and negative feedbacks (Kolb et al. 2016, Huang et al. 2020). In general, numerous North American cases show that drought increases bark beetle outbreak risk (Kolb et al. 2016).
The mean periodic thermal sum of April to September was used as a SBB outbreak risk proxy. It is known that springtime swarming conditions and timing and its subsequent effect on diapause, sister broods, and voltinism of SBB has an impact on the reproduction of SBB populations (Baier et al. 2007; Jönsson et al. 2011; Öhrn et al. 2014). Because of substantial annual variation in spring weather, the inclusion of swarming conditions is particularly important if the aim is to analyse year to year variation in reproductive success. However, the overall positive relationship of annual temperature sum and number of warm days in spring (Romashkin et al. 2020) suggest that climate warming will generally favour earlier swarming of SBB in NBSA. Moreover, the annual precipitation will increase in winter, but the solar radiation will not change much in April to May (Ruosteenoja et al. 2016a). This suggests that swarming will be promoted by earlier springs with favourable temperatures more frequently in the future than in the current climate.
Large-scale storm damages are likely to occur throughout the NBSA in future, and a shorter cold period when soil is frozen will further increase risk of uprooting of trees (Peltola et al. 2010; Venäläinen et al. 2020). Marini et al. (2017) concluded that storms as driving factors of SBB epidemics are decoupled from the two other drivers of temperature and drought. However, as the mean temperature sums rise well above 1500 DD in NBSA, it is very probable that after large scale storms, there will be a more rapid change from endemic phase to epidemic phase in population dynamics of SBB.
Regional trends in future climatic conditions favouring SBB epidemics in NBSA
In their benchmark study, Jönsson et al. (2011) predicted that the populations of SBB in the Baltic countries, Southern Sweden, and southernmost Finland will change from univoltine population dynamics to bivoltine by the end of the 21st century. However, the more recent climate models used in this analysis suggest that the change from univoltine to bivoltine population dynamics has already occurred in the Baltic Countries and Southern Sweden. Southernmost Finland will also follow soon, regardless of the RCP climate scenario used in the modelling. This is supported by the simulations, recent field observations (Kosunen et al. 2019; Pouttu and Annila 2010; Schroeder 2019; Wulff and Roberge 2020), and meteorological data (ECA&D 2021).
The future rise of temperature sums is evident, which predicts the increase of reproductive capacity of SBB. The probability of severe droughts, which decrease the ability of spruce to defend itself against SBB attacks, are higher in RCP4.5 and RCP8.5 scenarios than in RCP2.6, which is not surprising. From the point of view of forest health, an increase in temperatures in the RCP2.6 scenario suggests a slightly higher future outbreak epidemic risk in NBSA compared to the current situation. However, both RCP4.5 and RCP8.5 scenarios predict significant escalation and worsening of SBB epidemic risk in the latter half of the 21st century. The following paragraphs outline the potential development of the SBB epidemy risk in different regions of NBSA for different climate change scenarios.
East Central (EC) Sweden
In the best scenario (RCP2.6), the rapid increase of thermal sum and decrease of HTI will stop between 2020–2050. However, both remain at a more dangerous level compared to the historic period. This suggests that the SBB outbreaks continue to be a significant threat to the health of spruce forests throughout the 21st century in EC Sweden. After periods of dry and warm years, drought-driven SBB epidemics similar to those seen recently will occur (Schroeder 2019; Wulff and Roberge 2020). The realisation of RCP4.5 and RCP8.5 CC scenarios would predict even higher outbreak risk and wider scale damage than observed so far. In the RCP8.5 scenario, at the end of 2100, the thermal sum will cross the threshold of 2250 DD, potentially allowing the trivoltine population dynamics of SBB.
Southeastern (SE) Estonia
SE Estonia is in the same latitude as EC Sweden. Therefore, the thermal sum follows a similar pathway in SE Estonia and EC Sweden. However, the higher precipitation found in SE Estonia lowers the risk of droughts compared to that in EC Sweden. Because of high precipitation, the drought stress risk (HTI) remains moderate in RCP4.5 throughout 2100 and is high only in the RCP8.5 scenario after 2040. Furthermore, in RCP4.5 and RCP8.5 scenarios, increasing temperature sum put extra pressure on outbreak risk. In the RCP8.5 scenario, at the end of 2100, the thermal sum will cross the threshold of 2250 DD, also potentially allowing the trivoltine population dynamics of SBB.
Southern (S) Finland
In S Finland, the thermal sums are lower than in SE Estonia, but on the other hand, precipitation is slightly lower. Therefore, the risk of drought stress in different climate change scenarios is similar to that in SE Estonia. However, the low thermal sums suggest that the overall risk for future epidemics is lower in S Finland than in SE Estonia. In general, the thermal sums increase in Finland in all climate change scenarios, making the future conditions favourable for an increase in SBB reproduction (including change from univoltine to bivoltine population dynamics) and a higher risk of outbreaks.